This invention relates to an optical pickup head apparatus which can be used for recording and reproducing or erasing optical information on and from an optical or opto-magnetic medium.
The optical memory technique using a pit-like pattern as a high-density and high-capacity storage medium has been practiced by finding many applications such as digital audio disk, video disk, document file disk and data file. The optical pickup head (hereinafter abbreviated as OPUH) apparatus is based on three functional components, i.e., (i) an imaging optics for forming a diffraction-limited micro-spot, (ii) a component for effecting focusing of the optics and tracking error signal detection as well as pit-signal detection, and (iii) an actuator. In the past, advanced aspherical lens design and highly precise press work technologies have contributed to remarkable size-reduction and weight-reduction of the functional component in item (i) above and consequently compactness and high performance of the actuator in item (iii) have promisingly been insured. As regards the component for error signal detection in item (ii), however, even a signal detection component for use in an OPUH apparatus dedicated to reproduction and capable of being constituted with the simplest optical system must comprise a beam splitting means, a focusing control means based on an astigmatic aberration method or a knife edge method and a tracking control means in independent form or combined form. Optical parts such as beam splitter, lens and prism to be used conventionally in the signal detection component are difficult to manufacture in mass production, to assemble and to adjust and have disadvantages from the standpoint of size reduction, cost reduction, mass production and reliability.
Recently, a countermeasure to solve the above problems has been reported in "High Performance Optical Head using Optimized Holographic Optical Element" by Y. Kimura et al, Proc. of the International Symposium on Optical Memory, Tokyo, Sept. 16-18, 1987 (p. 131). In this proposal, an optical element having a complex function is introduced to solve the aforementioned problems and as shown in FIG. 1b in the accompanying drawings of the present application, a hologram element 16 is disposed near a focusing lens 3. Conventionally, when a hologram element prepared using light of a wavelength .lambda..sub.1 (400 to 500 nm) suitable for hologram recording is irradiated, for reconstruction, with a near infrared or red laser beam of a wavelength .lambda..sub.2 (.about.800 nm or 633 nm) suitable for a light source of OPUH apparatus, the lens action of the hologram suffers from a noticeable aberration which is difficult to correct. Under the circumstances, the hologram element is generated by means of a computer under design consideration based on a so-called Fourier transform holographic system which is an optics model as shown in FIG. 1a wherein an interference fringe pattern due to interference of two points P.sub.1 and P.sub.2 with a reference beam R is formed or an .epsilon.-.eta. plane or hologram surface (practically, an interference fringe pattern due to wave fronts 230 and 231 is formed on one half of the hologram surface and an interference fringe pattern due to wave fronts 230 and 232 is formed on the other half). Thus, the hologram element 16 used in FIG. 1b is prepared by electron beam graphics and in particular, has two domains 161 and 162 to attain effects equivalent to those of "wedge-prism method" or "double-knife edge method". In an optical head optics shown in FIG. 1b, a laser beam such as infrared beam or visual beam emitted from a laser source 1 passes through the hologram element 16 and lens 3 and irradiates the surface of an optical disk 4. A reflected beam from the optical disk passes through the lens 3 and element 16, whereby the diffracted beams are detected by a beam detector 15 so as to be used for focusing control, tracking control and the like. Light spots incident upon the photodetector 15 are diagrammatically shown in FIG. 1c. The hologram element prepared in the above manner can advantageously act as an aberration-free hologram lens for a wavelength limited to the design wavelength .lambda..sub.2 of the laser source 1 used and even in the event that an aberration due to a slight variation in wavelength of the laser source appears as the beam shift on the photoelectric conversion surface of the beam detector or photodetector 15, four photoelectric conversion domains 151, 152, 153 and 154 which can conveniently be operated in push-pull fashion suppress the varriation to a practically satisfactory extent.
However, the prior art optics using the hologram element 16 is disadvantageous in that in order to focus a small spot upon the linear domain-boundary (B in FIG. 1c) of the photodetector 15, the position of the spot has to be adjusted correctly within a range of .+-. several microns relative to the photodetector 15 as in the case of the usual optical system. In an astigmatic aberration type system in which astigmatic aberration wave fronts are detected by a four-domain or -element (quadrant) detector 15 as shown in FIG. 1c, the same difficulties are encountered in performing accurate final adjustment of the photodetector 15. In this connection, one may refer to, for example, "A Multi-functional Reflection Type Grating Lens for the CD Optical Head" by K. Tatsumi et al, Proc. of the International Symposium on Optical Memory, Tokyo, Sept. 16-18, 1987 (p. 127) and U.S. Pat. No. 4,731,722 to Wai-Hon Lee entitled "Optical head using hlogram lens for both beam splitting and focus error detection functions" issued May, 1988.
Further, in making an attempt to integrally form the light source (semiconductor laser) and the photodetector in the conventional OPUH apparatus using the holographic element, the position of the photodetector relative to the light source must also be adjusted with high accuracy.
In the conventional optics using a diffraction grating (knife edge method), an unwanted diffraction beam is produced from the diffraction element 16 irradiated with the light source 1, though not illustrated in the conceptive diagram of FIG. 1b, and focused by the lens 3 upon the surface of the optical disk 4 to form an unwanted diffraction spot image. The unwanted spot is a small spot like the 0-order diffraction beam spot and in a write-once type or erasable type optical disk, its intensity is too high to be neglected and degrades the S/N ratio upon signal recording or reading.